add comment for envelope converging expand calculation

parquet-column/src/main/java/org/apache/parquet/column/statistics/geometry/EnvelopeCovering.java

@@ -49,8 +49,17 @@ void update(Geometry geom) {
         Geometry existingGeometry = reader.read(geometry.array());
         Envelope existingEnvelope = existingGeometry.getEnvelopeInternal();
         Envelope newEnvelope = geom.getEnvelopeInternal();
+
+        // The following expandToInclude method works correctly for planar coordinate systems.
+        // It simply extends the existing envelope to include the new envelope by adjusting
+        // the min and max values of x and y coordinates.
         existingEnvelope.expandToInclude(newEnvelope);
 
+        // However, this approach is not accurate for spherical coordinate systems.
+        // In spherical coordinates, the Earth's curvature needs to be taken into account,
+        // and simple min/max bounds may not correctly represent the spatial extent.
+        // More sophisticated geodetic calculations are required for spherical envelopes.
+
         Geometry envelopePolygon = createPolygonFromEnvelope(existingEnvelope);
 
         geometry = ByteBuffer.wrap(writer.write(envelopePolygon));